Image signal coder operating at reduced spatial resolution

ABSTRACT

An image processing system operates at reduced resolution to reduce computational complexity while remaining fully compatible with full resolution decoders. A video input signal is subsampled and encoded at the resulting lower resolution. The encoded signal is filled with zero terms to produce an encoded signal having the same number of terms as a full resolution encoded signal. In a motion-compensated hybrid coder, the decoder section also includes a subsampling system, so that the reconstructed video signal is produced at the same resulting lower resolution. The encoder section and the decoder section are each inverse functions of the other, eliminating a drift problem associated with prior systems.

This application is a continuation of application Ser. No. 08/504,631,filed Jun. 7, 1995, entitled IMAGE SIGNAL CODER OPERATING AT REDUCEDSPATIAL RESOLUTION, and now U.S. Pat. No. 5,854,858.

FIELD OF THE INVENTION

The present invention relates generally to the field of digital imageprocessing systems, in particular image processing systems embodied incombinations of general or special purpose hardware and software. Morespecifically, the present invention relates to implementations of theDiscrete Cosine Transform suitable for embodiment as a software programfor processing moving images, i.e. video, executing on a general purposecomputer.

BACKGROUND OF THE INVENTION

In the context of the present invention, an image is a two-dimensionalvisual representation, wherein each point within the image may haveassociated therewith one or more characteristics. For example, for amonochrome image, each point may have associated therewith a luminancevalue. For a color image, each point may have associated therewith a redintensity, a blue intensity and a green intensity. Common imagepresentation technologies include printed photographic still images,movie images, television images, and computer images. Computertechnology has now begun to open whole new areas of image presentation,such as high realism video games, electronic books, and others yet toreach commercialization. These later forms of image presentation all usedigital images. That is, images which are stored in digital, and usuallybinary, form.

Digital image signals are formed by first dividing a two-dimensionalimage into a grid. Each picture element, or pixel, in the grid hasassociated therewith a number of visual characteristics, such asbrightness and color. These characteristics are converted into numericform. The digital image signal is then formed by assembling the numbersassociated with each pixel in the image in a sequence which can beinterpreted by a receiver of the digital image signal.

One reason that these emerging technologies have not appeared sooner isthat uncompressed digital image signals contain vast amounts ofinformation, requiring vast quantities of storage space. Furthermore,moving uncompressed digital image signals from one user to anotherrequires a large communication bandwidth to accommodate the large amountof information in a reasonable period of time. Suppose that for amonochromatic (e.g., black and white) image 256 shades of gray aresufficient to represent a uniform luminance scale ranging from black towhite. Each pixel occupies eight bits (binary digits) of storage. Thusan image created for display on a typical personal computer screenhaving a resolution of 640×480 pixels occupies a total of 307,200 bytes.That is the storage equivalent of approximately 100 pages ofsingle-spaced text. Extrapolating, a color image can occupy three timesthat storage space.

In view of the tremendous pressure that the use of images places onstorage requirements, there has been a great deal of research into imagecompression techniques. A standard known as ISO 10918-1 JPEG DraftInternational Standard/CCITT Recommendation T.81 has emerged as a resultof this research. The standard is reproduced in Pennebaker and Mitchell,"JPEG: Still Image Data Compression Standard," New York, Van NostrandReinhold, 1993, incorporated herein by reference. One compressiontechnique defined in the JPEG standard, as well as other emergingcompression standards, is Discrete Cosine Transform (DCT) coding. Imagescompressed using DCT coding are decompressed using an inverse transformknown as the inverse DCT (IDCT). An excellent general reference on DCTsis Rao and Yip, "Discrete Cosine Transform," New York, Academic Press,1990, incorporated herein by reference. It will be assumed that those ofordinary skill in this art are familiar with the contents of theabove-referenced books.

It is readily apparent that if still images present storage problems forcomputer users and others, motion picture storage problems are far moresevere, because full-motion video may require up to 60 images for eachsecond of displayed motion pictures. Therefore, motion picturecompression techniques have been the subject of yet further developmentand standardization activity. Important standards include ITU-TRecommendations H.261, H.262 and H.263. The ITU-T Recommendation H.262is commonly known as the MPEG standard, after the Motion Picture ExpertsGroup which developed it. These standards rely in part on DCT coding andIDCT decoding.

The DCT is applied, in accordance with these standards, to each image orvideo frame in a blockwise fashion. Block sizes that are powers of 2 (2,4, 8, 16, etc.) are particularly suitable for computationallyattractive, fast algorithms for the DCT. In practice, a block size of8×8 is almost always used today for image coding.

It will be apparent to those skilled in this art that the DCT is closelyrelated to the discrete Fourier transform (DFT). In fact, the DCT can beinterpreted as a DFT of the extended block of size 2N×2N, which isobtained by mirroring the original N×N block at its horizontal and itsvertical edge. Thus, the DCT coefficient can be interpreted as spectralcomponents of an image block. Low order DCT coefficients correspond tolow frequency components in the signal, while high order DCTcoefficients correspond to high frequency components.

Given that processing digital image signals using DCT coding providesthe desired degree of compression, the pressure on industry is now tofind the fastest method by which to perform the DCT and IDCT. As in thefield of compression generally, research is highly active andcompetitive in the field of fast DCT and fast IDCT implementation.Researchers have made a wide variety of attempts to exploit thestrengths of the hardware intended to implement the DCT and IDCT byexploiting symmetries and other properties found in the transform andinverse transform, as it is used in practical systems. For example, theApplicants' own method and apparatus disclosed in their U.S. patentapplication Ser. No. 08/332,535, filed Oct. 31, 1994, pending,incorporated herein by reference, exploits the statistical properties ofthe transformed signal.

Sometimes, as discussed therein, the image signal does not require thefull spatial resolution that is provided by a DCT based coding scheme.This is often true for the color difference signals. In many codingstandards, the color difference signals are transmitted at nominallyhalf the horizontal and vertical resolution compared to the luminancesignals. For many natural scenes, a spatial resolution of one quarter ofthat of the luminance signal, both horizontally and vertically, would besufficient. Consequently, the bandwidth of the color difference signalscan be reduced by filtering, and the higher order coefficients of a DCTapplied to this signal would be very small or even zero.

The numbers of computations required to perform a DCT increases with itsorder N. For N a power of 2, the computational complexity (i.e. thenumber of multiplications and additions) of the fastest DCT algorithmsis proportional to N×log(N). Hence, a one-dimensional DCT of order N=8is roughly 3 times as complex as a DCT of order N=4. For a 2D DCT, an8×8 DCT is roughly 9 times as complex as a DCT of order 4×4.

It is desired to implement these functions in software, because to do soreduces hardware costs. Specialized hardware embodying a softwareDCT/IDCT could be made more flexible than an all-hardwareimplementation. Software which could run on a conventional PC, withoutspecial hardware, could eliminate the cost of such hardware entirely.This may be especially advantageous in fields such as videoteleconferencing, where the participants are already likely to haveaccess to PCs. A video teleconference system could be implemented at afraction of the cost of current special-purpose hardware. Unfortunately,fast software DCT and IDCT implementations continue to suffer, relativeto their hardware cousins, due to the unusual demands placed on thecomputer by the required arithmetic operations, particularlymultiplications.

SUMMARY OF THE INVENTION

It is one object of this invention to reduce the computationalrequirements for a DCT coder by filtering and subsampling the originalimage signal and utilizing a DCT of lower order, when the full nominalresolution provided by a coding standard (and assumed by the decoder) isnot required. Other objects and advantages of the present invention willbecome apparent on reading the following summary and descriptions.

In accordance with one aspect of the invention, there is provided adigital signal encoder. The encoder includes a subsampling system forreceiving a digital image signal and producing a subsampled signal at anoutput; a signal transformer operating at a reduced spatial resolutioncorresponding to the subsampled signal, having an input connected toreceive the subsampled signal and an output carrying a transformedsignal; and a quantizer having an input connected to receive thetransformed signal and an output carrying a quantized output signal. Thesignal transformer may perform, for example a 4×4 discrete cosinetransform (DCT), or may perform other signal transforms, as mentionedbelow.

In accordance with another aspect of the invention, there is provided amotion-compensated digital signal encoder. The motion-compensateddigital signal encoder is of a hybrid encoder type, comprising a firstsubsampling system for receiving a digital image signal and producing asubsampled signal at an output; a subtractor having a first inputconnected to receive the subsampled signal, a second input connected toreceive a subsampled reconstructed video signal and an output carrying adifference signal representative of the difference between the signalsreceived at the first and second inputs; a signal transformer operatingat a reduced spatial resolution corresponding to the subsampled signal,having an input connected to receive the subsampled signal and an outputcarrying a transformed signal; a quantizer having an input connected toreceive the transformed signal and an output carrying a quantized outputsignal; a decoder having an input connected to receive the quantizedoutput signal and an output carrying a reconstructed video signal; and asecond subsampling system having an input connected to receive thereconstructed video signal and an output carrying a subsampledreconstructed video signal. Here, the transform performed may also be a4×4 DCT, for example. In that case, the decoder may include an inversetransform system, for performing an inverse discrete cosine transform(IDCT).

In accordance with yet another aspect of the present invention, a methodof processing video signals in a computer executing a software program,may include the steps of:

receiving a video input signal at an original resolution;

low-pass filtering the received signal;

subsampling the low-pass filtered signal to produce a lower resolutionsignal;

performing a transform on the lower resolution signal;

quantizing the transformed signal; and

producing a quantized signal at the original resolution by filling thequantized signal with zeros.

Another method of processing video signals in a computer executing asoftware program, in accordance with yet another aspect of the inventionincludes the steps of:

receiving a video input signal at an original resolution;

low-pass filtering the received signal;

subsampling the low-pass filtered signal to produce a lower resolutionsignal;

forming a difference signal between the lower resolution signal and alower resolution reconstructed video signal;

performing a transform on the difference signal formed;

quantizing the transformed signal;

producing a quantized signal at the original resolution by filling thequantized signal with zeros;

performing an inverse transform on the quantized signal at the originalresolution;

adding to the inverse transformed signal a predicted frame signal toform a predictor input signal;

forming the predicted frame signal from the predictor input signal; and

subsampling the predicted frame signal to form the reconstructed videosignal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, in which like reference numerals denote like elements:

FIG. 1A is a block diagram of an intraframe encoder/decoder system ofthe prior art;

FIG. 1B is a block diagram of an intraframe encoder/decoder system inaccordance with one aspect of the present invention;

FIG. 2 is a block diagram of a predictive encoder/decoder system inaccordance with another aspect of the present invention;

FIG. 3 is a block diagram of a predictive encoder/decoder system inaccordance with yet another aspect of the present invention;

FIG. 4 is a flow chart of a method embodying aspects of the invention;

FIGS. 5A and 5B are a flow chart of another method embodying aspects ofthe invention; and

FIG. 6 is a block diagram of a typical general purpose computer systemin which the invention may be practiced.

DETAILED DESCRIPTION

The invention will be better understood by reading the followingdetailed description of various embodiments thereof, in connection withthe Figures.

Conventionally, an intraframe encoder, as shown in FIG. 1A, receives avideo signal 1001, performs an 8×8 DCT 1003 and quantizes 1005 theresulting transform coefficients 1007. The quantized transformcoefficients are encoded and transmitted as signal 1009. At the decoder,an 8×8 IDCT 1011 is applied to the received signal 1009, to reconstructthe video signal, producing video out 1013.

Many video input signals and many applications do not require that thefull spatial resolution of the signal be encoded, to producesatisfactory video out upon subsequent decoding. If the full spatialresolution of the video signal is not required, computation can be savedas shown in FIG. 1B. The video signal 1001 is spatially filtered 1015and subsampled 1017, for example by 2:1 in both horizontal and verticaldirection. It will be readily apparent to those skilled in this art howthe following discussion may be modified if a ratio other than 2:1 isused. The subsampled signal 1019 is transformed 1021 in a blockwisefashion, using a 4×4 DCT, rather than an 8×8 DCT. The 4×4 blocks in thesubsampled image coincide with the 8×8 blocks that would have been usedin the full resolution image. For example, the block may include everyother pixel in each of the horizontal and the vertical directions. Theresulting 4×4 transform coefficients are then encoded as the 4×4 lowestorder transform coefficients of the 8×8 block. The remaining 48coefficients of the 8×8 block not produced as part of the 4×4 DCT areset to, and encoded as, zero, for example by quantizer 1023. The signaltransformer, which here performs a 4×4 DCT may be implemented using anyknown means, including specialized hardware or one of the methods andapparatus disclosed in U.S. patent application Ser. No. 08/332,535indicated above. The decoder in FIG. 1B is identical to that in FIG. 1A.Since the 4×4 DCT is computationally much less demanding than an 8×8DCT, the embodiment of this aspect of the invention, as shown in FIG. 1Bis advantageous when full spatial resolution is not required.

The filter 1015 applied before subsampling 1017 in FIG. 1B should be alowpass filter with a half-pel and half-line delay. The simplest suchfilter has the 2D impulse response: ##EQU1## Thus, after subsampling1017, each sample is the average of 4 samples of the original image.This filter 1015 is readily embodied in specialized hardware in asoftware program executing on general purpose hardware by those skilledin the art.

The above discussion has applied to intraframe encoder/decoder systems,suitable for use in processing still images. Next, interframe predictiveencoder/decoder systems suitable for encoding moving images, i.e. video,are discussed.

Often, the DCT is used in the context of a motion-compensating hybridcoder as shown in FIG. 2. A prediction 201 is formed based on theprevious frame, and the prediction error 203 is encoded by an 8×8 DCT205. The encoder contains a replica 207 of the decoder, including an 8×8inverse discrete cosine transform (IDCT) 209. Such coding schemes areused in the ITU-T recommendations H.261, H.262, and H.263, mentionedabove.

Conventionally, before the coder, the video signal 211 passes throughone or more preprocessing stages 213. Preprocessing operations 213 mightinclude noise reduction, image sharpening, contrast and color correctionor gamma predistortion. In the system in FIG. 2, these operations wouldbe applied to full resolution video signals 211.

If the full resolution of the video signal is not required, the coder inaccordance with another aspect of the invention, as shown in FIG. 3 canbe used. Before preprocessing 213, the video signal 211 is filtered andsubsampled 215, for example by 2:1 horizontally and vertically. Furtherprocessing, such as the preprocessing stage 213, then proceeds at areduced resolution. The prediction signal 217 formed by the interframepredictor 219 is subsampled 221 as well. The prediction error signal 223(at reduced resolution) is then transformed by a 4×4 DCT 225, asoutlined in the first aspect of the invention. The 4×4 blocks in thesubsampled image coincide with the 8×8 blocks, that would have been usedin the full resolution image. The resulting 4×4 transform coefficientsare then encoded as the 4×4 lowest order transform coefficients of the8×8 block. As before, the remaining 48 coefficients of the 8×8 block notincluded in the 4×4 transformation are set to, and encoded as, zero, forexample in quantizer 227.

An 8×8 IDCT 209 is applied in the coder of FIG. 3, thus producing a fullresolution quantized prediction error signal 231 at its output.Effectively, the 8×8 IDCT 209 interpolates the subsampled predictionerror 223 at the input of the 4×4 DCT 225. The interframe predictor 219then works at full resolution. The coder therefore still contains anexact replica 207 of the decoder. The decoder for the coder shown inFIG. 3 is identical to the decoder in FIG. 2, thus the reducedresolution coder is fully compatible with the conventional decoder.

The system shown in FIG. 3 saves computation in three respects:

The preprocessing stage 213 processes one quarter of the samples.

The prediction error computation of the interframe predictor 219processes, i.e. performs subtractions on, only one quarter of thesamples.

A 4×4 DCT is used instead of an 8×8 DCT.

The spatial resolution provided by the coder is, of course, only onehalf of the full resolution horizontally and vertically. This method andapparatus is particularly suitable for the encoding of the colordifference signals Cr and Cb in the context of the H.261 codingstandard. There is little subjective degradation due to the reducedresolution of the color difference signals. On the other hand, thereduced resolution in the preprocessing stage allows the use ofsophisticated preprocessing algorithms otherwise prohibited by theircomputational complexity at full resolution.

Above, 2:1 subsampling of the input signal both horizontally andvertically has been used as an example. In general, any combination ofsubsampling factors horizontally and vertically is possible (forexample: 2:1 horizontally, 4:1 vertically). As indicated above, one ofordinary skill in this art would know that the relative sizes of theresulting blocks would be adjusted accordingly.

A variety of useful modifications to the invention are possible andcontemplated by the inventors. Various filters could be used beforesubsampling, including no filters at all. Other transforms, such as theSlant transform, the Walsh Hadamard transform, the Haar transform, orthe Lapped Orthogonal transform could be substituted for the 8×8 DCTtransform relative to which the invention has been described. Theinvention could be applied selectively, i.e., certain blocks arerepresented and processed at full resolution, while others arerepresented and processed at reduced resolution.

A method of processing a video signal using an intraframe coder inaccordance with aspects of this invention is now described in connectionwith FIG. 4. The method may be practiced using specialized hardware, orusing a software program executing on a general purpose computer, suchas illustrated in FIG. 6 and described below.

A video input signal is first received, step 401. The video input signalmay be analog or digital in origin. High frequency components of thevideo input signal are removed by low-pass filtering, step 403. If thevideo input signal is received in analog form, this low-pass filteringstep may be performed either before or after sampling and conversion(steps 405a or 405b) of the analog signal to digital form. If the videoinput signal is received in digital form, the low-pass filtering step403 may be embodied in a software program executing on a general purposecomputer. In this case, the video input signal is stored in memory, andthe filtering (step 403) is performed by selectively altering thecontents of the memory in accordance with known digital filteringtechniques. After the high frequency components of the video inputsignal have been removed or reduced by low-pass filtering (step 403),the video input signal may be subsampled step 407, without losingsignificant additional information. Sub-sampling (step 407) and itsresult is described in detail above. The sub-sampled signal, which mayreside in memory in the case of an embodiment using a general purposecomputer, then undergoes a 4×4 discrete cosine transform, step 409, andquantization, step 411. These steps (steps 409 and 411) may be combinedin some embodiments. The coefficients of the 4×4 DCT correspond to thelow order coefficients of an 8×8 DCT covering the same image area. Thus,in order to maintain compatibility with conventional decoders, duringthese last steps, and preferably as part of quantization (step 411), thecoefficients of the 8×8 DCT not produced by the 4×4 DCT are filled in(step 411 a) as zeros. The 64 coefficients of the 8×8 DCT thus producedare the result 413 of this method.

In accordance with other aspects of the invention, and with somemodification, the method may be applied to interframe predictive coders,as well as intraframe coders. This is illustrated in the flow chart ofFIGS. 5A and 5B.

Again, a video input signal is first received, step 501. The video inputsignal may be analog or digital in origin. High frequency components ofthe video input signal are removed by low-pass filtering, step 503. Ifthe video input signal is received in analog form, this low-passfiltering step 503 may be performed either before or after sampling andconversion (steps 505a or 505b) of the analog signal to digital form. Ifthe video input signal is received in digital form, low-pass filteringstep 503 may be embodied in a software program executing on a generalpurpose computer. In this case, the video input signal is stored inmemory, and the filtering is performed by selectively altering thecontents of the memory in accordance with known digital filteringtechniques. After the high frequency components of the video inputsignal have been removed or reduced by low-pass filtering (step 503),the video input signal may be sub-sampled (step 507), without losingsignificant additional information. Sub-sampling (step 507) and itsresult is described in detail above. The sub-sampled signal, which mayreside in memory in the case of an embodiment using a general purposecomputer, may optionally be preprocessed in step 508, using a variety offilters. Then, after forming a difference signal in step 5008 bysubtracting a similarly sub-sampled reconstructed video signal 5009, thedifference signal undergoes a 4×4 discrete cosine transform andquantization. The coefficients of the 4×4 DCT correspond to the loworder coefficients of an 8×8 DCT covering the same image area. Thus, inorder to maintain compatibility with conventional decoders, during theselast steps, and preferably as part of quantization, the coefficients ofthe 8×8 DCT not produced by the 4×4 DCT are filled in as zeros. The 64coefficients of the 8×8 DCT thus produced are the result 513 of thismethod.

In order to produce the reconstituted video signal 5008 needed at thestep of subtracting (step 5009), the following additional steps areperformed. An 8×8 inverse DCT (IDCT) (step 515) is performed on theresult signal, to form a full-resolution video signal. A predicted framesignal is added (step 517) to the full-resolution video signal to forman input to the interframe predictor. An interframe prediction (step519) is performed, which forms the predicted frame signal 521 for thenext frame. The predicted frame signal 521 is also sub-sampled in step523 to form the reconstructed video signal 5009, referred to above.

The above-described methods and apparatus may be embodied in a generalpurpose computer executing a software program, as indicated above. Theembodiments and variations disclosed herein are all intended to beimplemented in the form of software running on a personal computer (PC),configured substantially as shown in FIG. 6. However, the specifichardware configuration is not a limitation of the present invention.Indeed, the invention could be embodied in software running on othertypes of general-purpose computers, or embodied in special-purposeintegrated circuits. However, the PC configuration of FIG. 6 illustratesthe general hardware parameters suitable for practicing the invention,without unduly complicating the detailed explanation thereof.

Referring to FIG. 6, the PC 101 includes a central processing unit (CPU)103, memory 105, input/output (I/O) system 107 and mass storage 109.These are now briefly discussed, before turning to the software of thepresent invention.

The CPU 103 should preferrably be of a current, high-performance typesuch as the Intel 486 family, the Intel Pentium family, a DigitalEquipment Corp. Alpha, a MIPS R4000 or Motorola 68040 devices. Theminimum processing speed is not a critical parameter, but may be chosenby the skilled designer. Naturally, differences in processing speed willhave a direct effect on the performance of different systems.

Having sufficient quantities of memory 105 available in the PC 101 willalso have a direct effect on system performance. It is desirable to beable to hold sufficient quantities of a digital input signal and adigital output signal, as well as certain program structures, such aslook-up tables used in transforming the input signal into the outputsignal. The PC 101 therefore preferably includes several megabytes (Mbytes) of RAM, as well as other forms of memory, such as operatingsystem ROM, which is not important to this description. (1Mbytes=1,048,576 bytes.)

Data and digital signals of various kinds may be passed into or out ofPC 101 through I/O system 107. An extremely wide variety of devices maybe included in I/O system 107. The devices of I/O system 107 allowdigital image signals to be received into or transmitted out of PC 101.For example, I/O system 107 may include a modem 108a for sending digitalsignals or receiving digital signals or data from remote devices andcomputers. Such digital signals could include digital image signals.Another possible I/O device is a frame grabber 108b, which could be usedto introduce into PC 101 television or video images which have beenconverted to digital form. Of course, I/O system 107 may include suchconventional I/O devices as a monitor 108c, keyboard 108d and printer(not shown), which may be desirable.

Programs, data and digital signals which need to be stored on a morepermanent basis than permitted by the RAM portion of memory 105 arestored in a mass storage device 109. The mass storage device 109 may bea hard disk, an optical disk, a CD-ROM or any permanent storage devicehaving similar characteristics to the devices mentioned.

Image signals are received into the PC 101 through one of the devices ofI/O system 107. Once inside the PC 101, the image signals will have beenconverted to digital form. That is, they are digital image signals. Inthis embodiment, it is presumed that the digital image signals are to beeither stored in a mass storage device 109 or transmitted to anotherlocation via one of the I/O devices of the I/O system 107. It is furtherpresumed in this embodiment that subsequently the digital image signalsare to be used to operate a display device, for example the monitor108c, to display the image in its customary form. One example of this iswhere the PC 101 is part of a conferencing network where images are tobe exchanged, stored and displayed on a plurality of such machines.

When the invention is embodied as a software program executing on aPC101, various program steps are carried out, having the CPU103 performvarious operations on values in RAM, received by or output through thevarious devices of I/O system 107, etc. For example, the digital imagesignals received for processing as described above may be held as valuesin RAM.

The present invention has now been described in connection with a numberof specific embodiments thereof. However, numerous modifications whichare contemplated as falling within the scope of the present inventionshould now be apparent to those skilled in the art. Therefore, it isintended that the scope of the present invention be limited only by thescope of the claims appended hereto.

What is claimed is:
 1. A digital signal encoder operative on a digitalimage signal including at least one image frame organized into signalblocks, the encoder compatible with a digital signal decoder having apredetermined spatial resolution defined by a predetermined number oftransform coefficients in each transform block equal to a number ofsamples in each signal block, comprising:a subsampling system forreceiving the digital image signal and producing a subsampled signal atan output; a signal transformer operating on each signal block of animage frame at a reduced spatial resolution defined by a smaller numberthan the predetermined number of transform coefficients in eachtransform block, the reduced spatial resolution corresponding to thesubsampled signal, the signal transformer computing only the smallernumber of transform coefficients, and the signal transformer having aninput connected to receive the subsampled signal and an output carryinga transformed signal including a transform block corresponding to eachsignal block; a quantizer having an input connected to receive thetransformed signal and an output carrying a quantized output signal; andmeans for encoding as zero those transform coefficients of afull-resolution transform corresponding to the reduced spatialresolution transform, but not computed by the signal transformer.
 2. Amotion-compensated digital image signal encoder compatible with adecoder have a predetermined spatial resolution defined by apredetermined number, M, of transform coefficients in each transformblock, comprising:a first subsampling system for receiving a digitalimage signal and producing a subsampled signal at an output; asubtractor having a first input connected to receive the subsampledsignal, a second input connected to receive a subsampled predictionsignal and an output carrying a difference signal representative of thedifference between the signals received at the first and second inputs;a signal transformer operating at a reduced spatial resolution definedby a number smaller than the predetermined number of transformcoefficients in each transform block, the reduced spatial resolutioncorresponding to the subsampled signal, having an input connected toreceive the subsampled signal and an output carrying a transformedsignal; a quantizer having an input connected to receive the transformedsignal and an output carrying a quantized output signal; means forencoding as zero those transform coefficients of a full-resolutiontransform corresponding to the reduced spatial resolution transform, butnot computed by the signal transformer; a decoder having an inputconnected to receive the quantized output signal and an output carryinga prediction signal; a second subsampling system having an inputconnected to receive the prediction signal and an output carrying thesubsampled prediction signal; a spatial lowpass filter having an inputadapted to receive the digital image signal and producing a filteredimage signal at an output wherein, the signal transformer performs atransform defined by N coefficients, N<M.
 3. A method of processingvideo signals in a computer executing a software program, comprising thesteps of:receiving a video input signal at an original resolutiondefined by blocks of M signal samples; subsampling the input signal toproduce a lower resolution signal defined by blocks of N signal samples,N<M; forming a difference signal between the lower resolution signal anda lower resolution prediction signal; performing a transform on theblocks of N signal samples of the difference signal formed; quantizingthe blocks of N signal samples of the transformed signal; producing aquantized signal including blocks of M signal samples at the originalresolution by filling the quantized signal with M-N zeros; performing aninverse transform on the quantized signal at the original resolution;adding to the inverse transformed signal an original resolutionprediction signal to form a predictor input signal; forming the originalresolution prediction signal from the predictor input signal;subsampling the original resolution prediction signal to form the lowerresolution prediction signal video signal; and filtering the digitalimage signal with a spatial lowpass filter.
 4. The digital signalencoder recited in claim 1, wherein the encoder further comprises:aspatial lowpass filter having an input adapted to receive the digitalimage signal and producing a filtered image signal at an output.
 5. Thedigital signal encoder recited in claim 4, the subsampler operating at a2:1 ratio horizontally and vertically.
 6. The digital signal encoderrecited in claim 5, wherein the lowpass filter includes a half-pel delayand a half-line delay.
 7. The digital signal encoder recited in claim 6,wherein the lowpass filter has a 2D impulse response: ##EQU2##
 8. Thedigital signal encoder recited in claim 1, wherein the signaltransformer performs a 4×4 discrete cosine transform.
 9. The digitalsignal encoder recited in claim 8, wherein the encoder is specializedhardware.
 10. The digital signal encoder recited in claim 8, wherein theencoder is a software program executing in general purpose hardware. 11.The digital signal encoder recited in claim 1, wherein the signaltransformer performs a Slant transform.
 12. The digital signal encoderrecited in claim 1, wherein the signal transformer performs a WalshHadamard transform.
 13. The digital signal encoder recited in claim 1,wherein the signal transformer performs a Haar transform.
 14. Thedigital signal encoder recited in claim 1, wherein the signaltransformer performs a Lapped Orthogonal transform.
 15. The digitalsignal encoder recited in claim 1, wherein the digital signal encoder isselectively applied to some digital image signal blocks.
 16. The digitalsignal encoder recited in claim 2, the subsampler operating at a 2:1ratio horizontally and vertically.
 17. The digital signal encoderrecited in claim 16, wherein the lowpass filter includes a half-peldelay and a half-line delay.
 18. The digital signal encoder recited inclaim 17, wherein the lowpass filter has a 2D impulse response:
 19. Thedigital signal encoder recited in claim 2, wherein the encoder isspecialized hardware.
 20. The digital signal encoder recited in claim 2,wherein the encoder is a software program executing in general purposehardware.
 21. The digital signal encoder recited in claim 2, furthercomprising: a preprocessor connected to receive the subsampled signalfrom the first subsampling system and having an output connected to thefirst input of the subtractor, applying the subsampled signal theretoafter preprocessing.
 22. The digital signal encoder recited in claim 2,wherein the interframe predictive decoder performs substantially thesame decoding as performed by the digital signal decoder having thepredetermined spatial resolution.